https://www.siliconera.com/sony-psn-store-will-drop-ps3-psp-vita-games-on-mobile-web-by-october-21-2020/
Reminder to this thread that PS3, PSP, Vita support is disappearing from the web store soon, with mobile following about a week later.
This is especially important for Vita, because the Vita store when accessed from the device itself doesn't show all the content in its alphabetical lists, doesn't always return results in searches, etc.
Your download list will still work, but the Vita download list is a list of *all* playstation store content, not just Vita. So you'll have to scroll, scroll, scroll, scroll until you get to the game you want. Then if you tap download, you have to return to the TOP of the list. This is a nightmare, especially for a game with some DLC. You scroll forever, tap download, get returned to the top, scroll forever gain, tap download for one item, repeat. Think Valkyrie Drive / Senran Kagura and how many different DLC items there are for hair, accessories, underwear, etc. If you wanted to install everything, it's very, very tedious.
I mostly kept a PS3 plugged in through the Vita gen and also downloaded everything to a PS3 whenever I bought it, but I'll be going through and checking it this weekend.. I also don't feel great about the longevity of a PS3 going forward so I'm also going to probably download every Vita game / DLC that isn't already on my device and back it up using QCMA (open source content manager program for Vita, that works with the built in Content Manager application on the Vita). Note that for those sealed games you never got around to, but had some free DLC you grabbed - the DLC installs into the game file I believe (not 100% sure on this, need to check), so unless you crack open the game, install the DLC, then back that up in QCMA you'll be missing the little DLC extras. You can queue them to a PS3 though.
Navigating the download list on the playstationstore website is also a bit rough. I highly recommend the PSDLE browser extension. It can give you one big, long list that you can browse and filter by PS4/PS3/PSP and whether you want PS+ included (another annoying thing about the download list on Vita is that if your PS+ ever expires, then you renew *ALL* those PS+ games get put at the top of the list).
You can also export your whole download list to a text file to keep track of what you own. If the PlaystationStore is removing the ability to browse your download list, PSDLE probably won't work after the update too. I'd recommend just saving a text file in case you ever want to reference. Worst case you can go through the download list on the vita itself and grab something you forgot later, and if you save the list in the default (date) order, it'll be in the same order as your download list. So if you were to scroll quickly, wait for the tiles to load, you could tell if you were before/after whatever you're looking for in this hypothetical if that makes sense..
PSDLE:
https://repod.github.io/psdle/
https://chrome.google.com/webstore/detail/psdle/jdjhhapoddhnimgdemnpbfagndcnmhii?hl=en
You can click on individual items in PSDLE and then click the Vita or PS3 button to queue downloads to your devices, but keep in mind that these devices have limits on how many they can have in their queue. PSDLE won't tell you if you queue too many, but if you go to the official download queue on the playstation store website under your account, you can see if you put too many and if some failed for that reason - you can also press retry here.
Edit: Sony possibly removed the download list/queue links under where you click your profile image on the browser store. Direct link still works, for now:
download list:
https://store.playstation.com/en-us/download/list
download queue:
https://store.playstation.com/en-us/download/queue
Anyway, plan of attack for me is:
1. Queue PS3/Vita stuff to PS3 since I don't have much to do there.
2. Save off PSDLE export for future reference of all PS3/PSVita/PSP content.
3. Browse all Vita content in PSDLE (either by date or alphabetically)
4. Whenever I encounter a game, also go grab all its DLC as in a few cases (Hatsune Miku Project Diva X for example) the DLC may not be alphabetically in the same place.
5. After queueing the downloads to Vita for the game + DLC (you can force it to grab it by restarting your Vita, same for PS3, if it isn't picking up the download from the web), install the DLC items
6. Back up the game with QCMA to a PC drive for safe keeping.
Then if I ever want to play something that wasn't installed/didn't fit on my memory card in the future, I could check what I should have for it - via the PSDLE export/backloggery/whatever, then just go grab it through content manager on my PC.
It's also a good time to grab any Vita / PS3 games you might've had your eye on. The PS3 store generally shows everything it has, at least, but it is slow. For Vita, as mentioned above, the store is awful and doesn't show everything. So if you really want something, the next few days may be the time to get it.
Edit 2: Final Notes after completion:
I've finished going through it myself, thought I'd share what I learned.
The Vita seems to only be able to handle queueing for install 14 items at a time. The rest will result in 'could not download messages'. These messages will populate first before the ones that can download. So if you queue 50 things and just see failure after failure, that's what's happening. After you install the 14, press 'retry' on the queue for the failed items, and restart the Vita + wait a few minutes for it to pull in from the queue again you'll get 14 more.'
PS3 is pretty similar. The limit is a lot higher, you may need to go back and press 'retry' and power cycle the PS3 several times to get through huge queues.
HIGHLY recommend making sure the "WiFi in Power Save" option is disabled under power save settings.
This setting is very counterintuitive. It SOUNDS like it means, "use WiFi while in sleep/power save mode". That's not what it does. What it actually does is "run WiFi in a mode that saves power". You can get worse performance on your downloads if you have this set.
That's about it.. It was a huge pain in the butt. Popping in cartridges/downloading digital games, downloading DLC 14 at a time.. Installing them, restart, get 14 more, repeat until done, and then copy over to PC. But it's done and it feels good to have a backup.
Valkyrie Drive Bikini Party Edition, Cross Edge, Mugen Souls, Mugen Souls Z, and the PS3 agarest games were the worst. They had SO freaking much free DLC I wanted to grab and download just to have.
At the very end, literally as I was writing this post and wanted to just transfer over a couple of Vita PS+ titles that I figured "may as well" - content manager freaked out on me. It wouldn't connect. I reconnected my cable, and when I did it said I had no content on my PC. Somehow EVERYTHING was wiped, after about two days spent doing this. Thankfully.. I have a 64GB card and -most- things were on there still, so all I have to do is copy again.
I think what happened is that the official content manager application opened at some point and QCMA fought with it.. I have no idea. I only have to download a few games again, but be careful. Before you delete something, or switch memory cards, you may want to make a manual backup of what QCMA stored just in case. But other than this one weird hiccup, it was smooth sailing for me. Just tedious.
ryza 1 yet. 
